There has been increased demand for electrically erasable and programmable semiconductor memory devices that do not require a refresh operation for retaining data. There has also been demand for an increased degree of integration with respect to semiconductor memory devices.
A flash memory device does not need a refresh operation to retain data. Since data may be retained even at power-off, flash memory devices may be widely used in electronic devices.
FIG. 1 is a cross-sectional view of a conventional flash memory cell. Referring now to FIG. 1, a conventional flash memory cell 100 includes a source 120, a drain 130, a floating gate 140, and a control gate 150. The source and drain 120 and 130 are formed in a P-type semiconductor substrate 110, with a channel region between the source 120 and the drain 130. The floating gate 140 is formed on the channel region with a thin insulation film therebetween. The control gate 150 is formed on the floating gate 140 with a thin insulation film therebetween.
The source 120, the drain 130, the control gate 150, and the substrate 110 are connected to corresponding terminals Vs, Vd, Vg, and Vb that are respectively supplied with required voltages for a program, erase or read operation. In a program operation, a program voltage may be applied to a control gate (i.e., a word line) of a selected flash memory cell, and a pass voltage lower than the program voltage may be applied to a control gate (i.e., a word line) of respective unselected flash memory cells. Methods for programming flash memory cells are disclosed in U.S. Pat. No. 5,473,563, the contents of which are hereby incorporated by reference.
In general, it may be difficult to sufficiently program a flash memory cell 100 to a target threshold voltage via one program loop. This means that the flash memory cell 100 may be programmed to have a target threshold voltage via plural program loops. Whether a flash memory cell is programmed may be determined according to the amount of charge accumulated at its floating gate.
Sufficient charge accumulation at a floating gate of a flash memory cell may enable its threshold voltage to be increased, which may result in no current flow between its drain and source. On the other hand, insufficient charge accumulation at a floating gate of a flash memory cell may enable current flow between its source and drain at a lower threshold voltage. Programming of a flash memory cell may be determined by detecting whether current flows via the flash memory cell. In other words, a flash memory cell may be determined to be programmed when a sufficient charge is accumulated at the floating gate to increase the threshold voltage of the flash memory cell. A flash memory cell may be determined not to be programmed when the charge accumulated at the floating gate is not sufficient to increase the threshold voltage of the flash memory cell.
A verify read operation is carried out to determine whether a flash memory cell is programmed. A wired-OR pass/fail check scheme is a known verify read method. In accordance with the wired-OR pass/fail check scheme, signals output from page buffers may be used as inputs of an OR gate, which outputs a detect signal as a pass/fail signal in response to input signals.
FIG. 2 is a block diagram showing a wired-OR pass/fail check circuit of a conventional non-volatile memory device. Referring to FIG. 2, fuses 23_1˜23_n are connected to output terminals of page buffers 22_1˜22_n. One fuse is connected to multiple page buffers, as illustrated in FIG. 2. A fuse may be cut when one of the bit lines connected to corresponding page buffers is defective. The fuses 23_1˜23_n may respectively be used as electrical isolation means.
A signal nWDO from each of the page buffers 22_1˜22_n indicates whether a corresponding flash memory cell is programmed normally. For example, when a signal nWDO has a logic high value, it may indicate that a memory cell connected to a corresponding page buffer is not programmed. When a signal nWDO has a logic low value, it may indicate that a memory cell connected to a corresponding page buffer is programmed.
Although not illustrated in FIG. 2, a path for outputting the signals nWDO from the page buffers 22_1˜22_n is different from a data input/output path. Each page buffer in the page buffer circuit 220 may include two latches (i.e., a main latch and a sub/auxiliary latch) to support a cache operation. In each page buffer, a main latch may be configured to drive a bit line with a power supply voltage or a ground voltage in response to input data, and a sub/auxiliary latch may be configured to load data in advance. The signals nWDO may be output in response to data stored in main latches of the page buffers 22_1˜22_n.
While the signals nWDO are transferred to the wired-OR pass/fail check circuit 240 from the main latches of the page buffers 22_1˜22_n, it may be possible to load next data to be programmed onto the sub/auxiliary latches of the page buffers 22_1˜22_n. This cache operation may enable a program speed to be improved.
A fuse corresponding to a defective bit line or page buffer may be cut in order to prevent a pass/fail check result from being affected by defective bit lines and/or page buffers. Whether a non-volatile semiconductor memory device includes defective bit lines may be tested prior to shipping. If defective bit lines are detected, fuses corresponding to defective bit lines may be cut. Cutting of fuses may be performed via various cutting techniques such as using a laser beam. Defective bit lines may thereby be isolated from normal bit lines after the corresponding fuses are cut.
With conventional design technologies, it may be difficult to provide each fuse within an area where one page buffer is arranged. For this reason, a layout of the fuses may be provided according to a design scheme where one fuse is connected to at least two page buffers, as illustrated in FIG. 2. This may also require a relatively wide area to arrange fuses, which may make it difficult to design high-density memory devices and/or to lower memory costs.
Accordingly, a Y-scan pass/fail check scheme has been proposed to perform a program verify operation without electrical isolation means such as fuses. The Y-scan pass/fail check scheme may perform a sequential scan of the columns (i.e., bit lines) of a selected page during a program verify operation.
FIG. 3 is a block diagram showing a Y-scan pass/fail check circuit 300 in a conventional non-volatile memory device.
Referring to FIG. 3, a column selector circuit 330 is connected to page buffers 32-1˜32_n in a page buffer circuit 320. Data for Y-scan pass/fail check may be output via a data input/output path. In other words, a data output path for a pass/fail check operation may be identical to a conventional data input/output path. For this reason, during a Y-scan pass/fail check operation, it may not be possible to load data onto sub/auxiliary latches of the page buffer circuit 320. The Y-scan pass/fail check scheme may require further time for loading onto a page buffer circuit 320 data to be programmed in a next page of a memory cell array 310. Unlike the wired-OR pass/fail check scheme, the Y-scan pass/fail check scheme may be made such that page buffers 32_1˜32_n of the page buffer circuit 320 are checked sequentially. This means that a program time may be increased when the Y-scan pass/fail check scheme is used, as compared with the wired-OR pass/fail check scheme.